1. Field Of The Invention
This invention is concerned in one aspect, with the preparation of novel molybdenum salts of dialkyldithiophosphoric acids and other organo-phosphorus acids by reaction of an aqueous molybdic acid solution with the desired phosphorus acid in the presence of an inert solvent mixed with a polar solvent, preferably ethyl acetate or other esters, and with their use as lubricating oil additives. In another aspect, the invention relates to reducing fuel consumption by adding these additives to the lubricants used.
2. Description of Prior Disclosures
There are numerous patents and literature references on molybdenum salts of dialkyldithiophosphoric acids and related acids. This background disclosure therefor is restricted to those which are believed most relevant.
Two patents, U.S. Pat. Nos. 3,400,140 and 3,494,866, disclose compounds [(RO).sub.2 PSS].sub.2 MO.sub.2 S.sub.2 O.sub.3 prepared by reaction of two moles of the phosphorus acid with one mole of molybdic acid (acidified aqueous Na.sub.2 Mo.sub.4 or ammonium molybdates) in the presence of a hydrocarbon as the only organic solvent. In the patented art, the products retain only one-half of the dialkyldithiophosphoric acid charged (i.e. the claimed composition has a Mo:P atomic ratio=1:1). Also, the products are claimed to possess a S:P ratio greater than the 2:1 in the charge; hence the increased S:P ratio must arise by transfer of the additional sulfur from the half of the acid which is not otherwise used in the reaction.
Patents cited in the above-mentioned patent application include U.S. Pat. Nos. 3,402,188; 4,202,781; 2,368,000; 2,806,022 and 2,962,493. Of these, the first discloses molybdenum salts of organophosphorus acids having the structural formula: ##STR1## wherein R is hydrocarbyl; X is sulfur or oxygen; a and b are integers with (a+2b) being 4 to 8. The patentee discloses various methods for preparing these salts including one involving the reaction of an organophosphorus acid with molybdenum trioxide or dioxide which is promoted by the addition of water or a lower alcohol. Indifferently, the reaction may be carried out by mixing the molybdenum compound with the acid in water and then adding water; they may be initially reacted in benzene followed by the addition of methanol; or the aqueous mixture after reaction is treated with naphtha and benzene.
U.S. Pat. No. 4,202,781 shows the preparation of molybdenum-phospho-sulfurized hydrocarbons by reacting the hydrocarbon optionally in a diluent inert hydrocarbon with an aqueous solution of a molybdenum compound and removing the molybdenum-depleted aqueous phase by distillation or partitioning. The patentee indicates that oxygen-containing solvents are limited as diluents for they cause emulsions owing to their poor solubility in the aqueous phase.
U.S. Pat. No. 2,368,000 describes the preparation of barium, calcium, zinc, aluminum and tin salts of dithiophosphoric acid esters by dissolving a diester in a low boiling mixture containing unstated proportions of non-polar and polar solvents, such as an alcohol and a hydrocarbon and adding to the resulting mixture a metal oxide. Similarly, U.S. Pat. No. 2,806,022 describes the preparation of neutralized reaction products of a phosphorus sulfide and a hydrocarbon with an alkali metal or alkaline earth metal compound wherein the neutralization is carried out in the presence of between 0.5 and 2 moles of water per mole of compound and 2 to 20 moles of an alkanol per mole of metal compound and heating to remove the water and alkanol. U.S. Pat. No. 2,962,493 improves on the previous patent by refluxing a hydrolyzed hydrocarbon-phosphorus sulfide with the above solvents prior to neutralization with the metal compound.
In contradistinction to the alkali and alkaline earth metal oxides employed in the above-discussed three patents, molybdenum oxides such as MoO.sub.3 react with acids only under the special conditions provided by very strong mineral acids and reducing acids. Molybdenum oxides are in fact acid anhydrides and react substantially with bases. Thus the chemistry of molybdenum oxides differs drastically from that of the heavy metal oxides such as Fe.sub.2 O.sub.3, PbO, ZnO, CaO, and BaO. This is known, for example from page 779 of "Advanced Inorganic Chemistry" by F. Albert Colton and B. Wilkenson, Interscience Publishers, John Wiley and Sons, 1962. Discussing the second transition series of elements, the authors state: "The most important of the oxides are MoO.sub.3 and WO.sub.3 . . . They are not attacked by acids but dissolve in bases to form molybdate and tungstate solutions . . . It (MoO.sub.3) is the anhydride of molybdic acid but it does not form hydrates directly."
The present invention is predicated on the unobvious discovery that molybdenum salts of certain organophosphorus acids, when prepared in a mixture of solvents and polar cosolvents present in a certain proportion have a higher conversion of molybdenum and phosphorus than is produced in the absence of such a mixture. The polar organic solvent functions to solubilize the molybdic acid (or other hydrous molybdenum oxides) into the organic phase. As a consequence, the subject products are compositionally different from those of the prior art in that the resulting molybdenum salts have Mo:P ratios which approach the ratio of the charge.
In the practice of the invention, an essentially 1:1 Mo:P ratio in the product may be attained without need to employ a sacrificial amount of excess phosphorus acid. Another difference from the prior art is that the water is removed by azeotropic distillation before recovering the product by filtration and stripping. These differences from the prior art result in a more convenient, lower cost process which minimizes the need to dispose of unreacted Mo and P into the environment.
As will be seen hereinafter, none of the prior art references known to the applicant discloses, hints or suggests in any manner whatsoever applicant's novel, unique and unobvious process and products.